The present invention relates to a tracked vehicle steering system, and more particularly, to a tracked vehicle steering system which has a steering wheel coupled to a variable friction device.
There are commercially available tracked vehicles with hydro-mechanical drive/steering systems which include an engine driven hydraulic steering pump which is responsive to steering pump control signals provided from a spring-centered steering wheel, and which drives a hydraulic steering motor. The steering motor drives a differential track drive mechanism which drives left and right tracks, and turns the vehicle at turning rates which depend on the magnitude of the steering pump control signals and engine speed. With such systems, the steering wheel position provides a direct indication of the position of swash plate of the pump, so the limit of pump capacity is coincidental with the limit of steering wheel rotation. While functional, such systems lack various features, and vehicles with such systems do not drive like cars or other familiar vehicles. For instance, though the position of the steering wheel indicates the swash plate position, the actual turn radius of the vehicle will depend on both the engine speed and ground speed.
A proposed electrohydraulic tracked vehicle drive/steering system has been described in U.S. Pat. 6,039,132, issued Mar. 21, 2000, and assigned to the assignee of this application. As described in U.S. Pat. No. 6,000,490, issued Dec. 14, 1999, and also assigned to the assignee of this application, there has also been proposed an electrohydraulic tracked vehicle drive/steering system which has a non-spring centered steering wheel coupled to a variable friction steering input device. The steering input device produces a variable, two-level friction force which resists turning of the steering wheel, provides feedback to the operator, simulatesxe2x80x9cend stopsxe2x80x9d on the steering wheel motion, and thus allows the tracked vehicles to drive more like wheeled vehicles. In this proposed system the higher friction level is turned on when a certain amount of steering wheel rotation is reached, similar to what occurs with a wheeled vehicle. But in higher gears of the vehicle transmission, the limit of steering pump stroke is reached before the corresponding amount of steering wheel rotation is reached. Thus, additional rotation of the steering wheel cannot cause a tighter turn. It would be desirable to provide the operator with an indication of when this condition is about to be reached.
Accordingly, an object of the present invention is to provide an electrohydraulic tracked vehicle steering system which provides the operator with an indication of when the limit of the steering pump stroke is reached when the transmission is in higher gear ratios.
These and other objects are achieved by the present invention, wherein a ratio of the desired or commanded steering motor speed to the engine speed is generated and compared to a limit value. When the ratio value reaches the limit, the limit of the steering pump is assumed to be imminent, and a variable friction steering input device coupled to the steering wheel is set to a high friction level. This indicates to the operator that the end of steering capability has been attained, and that a tighter turn radius should not be expected under such conditions. This is especially useful in a tracked vehicle with an electrohydraulic drive/steering system.